The conversion of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) to mevalonate is an early and rate-limiting step in the cholesterol biosynthetic pathway. This step is catalyzed by the enzyme HMG-CoA reductase. Statins inhibit HMG-CoA reductase from catalyzing this conversion. As such, statins are collectively potent lipid lowering agents.
Atorvastatin calcium is currently sold as Lipitor® having the chemical name [R—(R*,R*)]-2-(4-fluorophenyl)-β,δ-dihydroxy-5-(1-methylethyl)-3-phenyl-4-[(phenylamino)carbonyl]-1H-pyrrole-1-heptanoic acid calcium salt (2:1) trihydrate and the formula

Atorvastatin and pharmaceutically acceptable salts thereof are selective, competitive inhibitors of HMG-CoA reductase. As such, atorvastatin calcium is a potent lipid lowering compound and is thus useful as a hypolipidemic and/or hypocholesterolemic agent, as well as in the treatment of osteoporosis, benign prostatic hyperplasia (BPH), and Alzheimer's disease.
A number of patents have issued disclosing atorvastatin, formulations of atorvastatin, as well as processes and key intermediates for preparing atorvastatin. These include: U.S. Pat. Nos. 4,681,893; 5,273,995; 5,003,080; 5,097,045; 5,103,024; 5,124,482; 5,149,837; 5,155,251; 5,216,174; 5,245,047; 5,248,793; 5,280,126; 5,397,792; 5,342,952; 5,298,627; 5,446,054; 5,470,981; 5,489,690; 5,489,691; 5,510,488; 5,686,104; 5,998,633; 6,087,511; 6,126,971; 6,433,213; and 6,476,235, which are herein incorporated by reference.
Additionally, a number of published International Patent Applications and patents have disclosed crystalline forms of atorvastatin, as well as processes for preparing amorphous atorvastatin. These include: U.S. Pat. No. 5,969,156; U.S. Pat. Nos. 6,121,461; 6,605,759; WO 01/36384; WO 02/41834; WO 02/43667; WO 02/43732; WO 02/051804; WO 02/057228; WO 02/057229; WO 02/057274; WO 059087; WO 02/083637; WO 02/083638; WO 03/011826; WO 03/050085; WO 03/07072; and WO 04/022053.
It has been disclosed that the amorphous forms of a number of drugs exhibit different dissolution characteristics and in some cases different bioavailability patterns compared to the crystalline form (Konno T., Chem. Pharm. Bull., 1990;38:2003-2007). For some therapeutic indications one bioavailability pattern may be favored over another.
Variations in dissolution rates can make it advantageous to produce atorvastatin formulations in either crystalline or amorphous forms. For example, for some potential uses of atorvastatin (e.g., acute treatment of patients having strokes as described in Takemoto, M.; Node, K.; Nakagami, H.; Liao, Y.; Grimm, M.; Takemoto, Y.; Kitakaze, M.; Liao, J. K., Journal of Clinical Investigation, 2001; 108(10): 1429-1437) a rapid onset of activity may be highly beneficial in improving the efficacy of the atorvastatin.
The preparation of amorphous atorvastatin has been previously disclosed. For example, Lin et al., U.S. Pat. No. 6,087,511 disclose forming amorphous atorvastatin from crystalline atorvastatin. To form amorphous atorvastatin, Lin et al. disclose that crystalline atorvastatin is dissolved in a non-hydroxylic solvent such as tetrahydrofuran. The non-hydroxylic solvent is removed to produce a brittle foam that is broken up by mechanical agitation to afford amorphous atorvastatin.
WO 00/71116 also discloses forming amorphous atorvastatin using a non-hydroxylic solvent.
WO 01/28999 discloses a process for forming amorphous atorvastatin by recrystallization of crude atorvastatin from an organic solvent which comprises dissolving crude amorphous atorvastatin calcium in a lower alkanol containing 2-4 carbon atoms or a mixture of such alkanols under heating. The amorphous atorvastatin calcium is precipitated after cooling. WO 01/42209 discloses preparing amorphous atorvastatin by precipitating the atorvastatin using a solvent in which atorvastatin is insoluble or very slightly soluble, from a solution of atorvastatin which is provided with a solvent in which atorvastatin is freely soluble. Preferred solvents in which atorvastatin is freely soluble include low molecular weight alcohols, e.g. methanol and ethanol.
U.S. Pat. No. 6,531,507 B1 and U.S. 2003/0109584 A1 disclose HMG-CoA reductase inhibitors that are stabilized by forming a homogeneous composition with a buffering substance or basifying substance. The HMG-CoA reductase inhibitor and buffering substance or basifying substance are crystallized or co-precipitated from the same medium.
The current processes for production of amorphous atorvastatin involve solvents which are not optimal due to toxicity or environmental concerns. In addition, current processes are not optimal in terms of production capabilities. Therefore, there remains a continuing need for improved methods for preparation of amorphous atorvastatin.